Field of the Invention
This invention relates to the detection of a break or short circuit in a line such as a power supply line on power distribution systems. The distribution system can be AC or DC.
The invention can be adapted to isolation of power supply lines which have broken.
The invention is readily adapted for use in various power distribution systems, such as three phase, two phase, single phase, single wire earth return, whether bare wire or covered conductor distribution systems.
Description of Related Art
In AC power distribution systems, the power supply line can carry a higher supply voltage than the voltage supplied to the consumers. A transformer is used to reduce the power line voltage to the level required for the consumer. This is done to reduce the current on the power supply line, or, in the case of single line earth return systems, to provide sufficient voltage to overcome the high resistance of the earth return path. For example, in an SWER (single wire earth return) system, the power line voltage can be above 1000 v, while the customer voltage may be of the order of 250 v.
A covered conductor system is a high voltage distribution system using one or more lightly insulated overhead line conductors. SWER applications systems can be difficult to adequately protect due to the high impedance or resistance of the circuit and earthing arrangements. If the line breaks and the upstream side of the line falls to the ground or if a tree touches the line, there may be a delay before the standard earth fault protection systems operate due to the electrical insulation of the cable delaying or preventing detection of the fault current. If sparking occurs, this can start bush or brush fires and may thus result in damage. If there are no sparks, the line may remain live and present a risk of electrocution if a person or animal were to come into contact with the wire.
In the case of a bare overhead conductor system, such as for a bare wire SWER cable or other bare wire conductor systems, the broken cable may make sufficient contact with the ground to trip the Automatic Circuit Reclosure switch (ACR) or other earth leakage protection breaker switch. These earth fault protection systems now in use may not be sufficiently sensitive to operate reliably to detect earth faults, particularly where the current drawn by the consumer is low. Further, if a bare wire cable were to break and the upstream end (supply side) of the broken cable were too short to reach the ground or were to make inadequate contact with the ground, the existing fault detection system may not detect the break, leaving the upstream end of the cable live. This would give rise to a risk of electrocution if a person or animal were to come in contact with the live end of the cable.
It should be noted that unlike underground cable circuits most overhead line circuits whether they are bare or covered, are protected with an Automatic Circuit Reclosure Circuit Breaker System (ACR). The principle is that because overhead lines are be subjected to numerous transient faults e.g. birds sitting on insulators or windblown debris. Shorting out the wires. When such a fault occurs, the ACR first switches off the circuit but then automatically switches the power on again, usually after about a 1 second interval. If a fault is still apparent on the second energisation then it will then switch off the circuit. If no fault is detected on the re-energisation, then the power remains on the circuit. Naturally if a line breaks and the conductor is not shorted to earth then the ACR might create a dangerous situation because power will be allowed to remain on the circuit. Thus the line break protection system of the present invention can be used in conjunction with an ACR to prevent the reclosure of the ACR from delivering power to the broken wire.
Putting the supply underground would solve the problem but this is expensive using conventional HV ABC (aerial bundled cable) is costly and requires more poles due to the weight of the cable.
Power line communication (PLC) systems are used by power utilities to communicate between elements of the power supply system and consumer meters.
In the following description, the terms “upstream” and “downstream are used such that “upstream” refers to a location nearer the source of electricity supply, while the term “downstream” refers to a location further from the source of the source of supply of electric power. Similarly, “above” and “below” are used to refer to relative proximity or remoteness to the supply end of the distribution system, unless the context requires otherwise.